The present invention relates generally to a brake assembly, and in particular to a sliding caliper brake assembly or a brake assembly having a sliding disc for use on heavy road vehicles such as trucks, lorries, buses, etc.
Brake calipers for motorcycles and cars are relatively light and are therefore easily handled by a single person. Accordingly, when replacing worn brake pads, it is acceptable to provide a design such that the brake caliper is removed, the worn brake pads are then replaced with new brake pads, and then the brake caliper is replaced. Such an arrangement is shown in U.S. Pat. No. 4,200,173.
Sliding caliper brakes for heavy road vehicles are necessarily themselves heavy. Typically, the sliding caliper alone (in the absence of an associated carrier) will be so heavy that it is unsafe to be lifted by a single person alone (for example, during servicing) when the worn brake pads are replaced with new brake pads. For this reason, known heavy vehicle sliding caliper brake assemblies include two circumferentially spaced bridge arms each connecting the actuator side of the caliper to the reaction side, thereby defining a window through which a worn brake pad can be removed and through which new brake pads can be inserted. Such an arrangement does not require the removing of the sliding caliper. EP0906856 shows just such an arrangement.
It is advantageous for manufacturers to minimize the number of different parts for brake assemblies in order to save on tooling costs and in order to gain the benefit of high volume production. Furthermore, by designing a caliper with a fewer number of different parts, this provides benefits in terms of stocking spares. It is particularly advantageous to minimize the number of different parts for service items, i.e., items that will normally require replacement at appropriate service intervals throughout the life of the associated vehicle. Accordingly, brake manufacturers will often design brake assembles wherein the two brake pads for the brake assembly are identical. When considering a vehicle axle, the caliper on the right hand end of the axle is often a mirror image of the caliper on the left hand end of the axle, meaning that it is possible for all four brake pads on that axle to be identical. U.S. Pat. No. 4,200,173, EP0906856 and EP0752541 are examples of brake calipers including a pair of brake pads which are identical. Accordingly, the brake pads are interchangeable. Thus, a particular brake pad can be fitted on either the inboard side or the outboard side of the brake disc with the other brake pad being fitted on the other of the inboard side or outboard side of the brake disc.
By contrast, U.S. Pat. No. 5,343,985 shows an example where the outboard single brake pad is not interchangeable with the pair of smaller inboard brake pads.
The brake pads of brake calipers can either be actuated from both sides of the caliper or they can be actuated from a single side. An example of the former would be where a brake caliper includes one or more hydraulically operated pistons on an inboard side and the same number of hydraulically operated pistons on an outboard side. Under such circumstances, the caliper is axially fixed to a suspension component, and the brake disc is also axially fixed to another suspension component. Actuation of the brake will cause hydraulic fluid to move the inboard piston or pistons outwardly, thereby pushing the inboard brake pad onto the brake disc and causing the outboard piston or pistons to move inwardly, thereby pushing the outboard brake pad onto the brake disc.
In contrast, U.S. Pat. No. 4,200,173, EP0906856, EP0752541 and U.S. Pat. No. 5,343,985 show examples of brake calipers that are actuated from a single side. In all cases, the actuator will be mounted on the inboard side of the brake disc. Since none of these examples have an actuator on the outboard side of the brake disc, the outboard side of the caliper is relatively compact and therefore occupies a relatively small space envelope within the wheel than would be the case for an equivalent caliper having an actuator on an outboard side.
U.S. Pat. No. 4,200,173, EP0906856, EP0752541 and U.S. Pat. No. 5,343,985 are all examples of sliding caliper brakes. Thus, the calipers will be allowed to slide, typically on pins, in an axial direction (when considering the axis of rotation of the associated brake disc). This allows for the brake caliper to move slightly inboard when the brake is applied to allow the running clearance between the brake disc and the outboard brake pad to be closed so as to frictionally engage the outboard brake pad with the brake disc. Furthermore, as the outboard brake pad wears, the normal running position of the caliper (i.e., the position of the caliper when the brakes are not applied) will progressively move axially inwardly to maintain the running clearance between the outboard brake pad and the disc.
It is also possible to have a caliper actuated from a single side in which the caliper is axially fixed to a suspension component, as opposed to the “sliding caliper” design refer to above. However, a caliper that is actuated from a single side and is axially fixed requires a sliding brake disc. In order to take up the running clearance between the outboard brake pad and the disc, the disc must be moved towards the outboard brake pad since the outboard brake pad is axially fixed in the caliper and the caliper, as mentioned above, is also axially fixed. Sliding disc brakes having actuators actuated from a single side are known.
The brake pads of known brake calipers include a steel brake pad backplate onto which is bonded or otherwise affixed friction material. The friction material is typically arcuate in shape in order to correspond with the brake disc against which it engages during braking (see, for example, U.S. Pat. No. 4,200,173, EP0906856, EP0752541 and U.S. Pat. No. 5,343,985). The brake pad backplate is therefore also generally arcuate in shape and has circumferential ends. When the brakes are applied, the braking torque loads (also known as tangential loads) must be transferred from the brake pad to the vehicle via the suspension. U.S. Pat. No. 4,200,173 and EP0752541 show examples of the braking torque loads of both the inboard and outboard brake pads being transferred to a brake carrier that is rigidly fixed to the vehicle axle or other equivalent suspension component. Significantly, the braking torque loads are not transferred through the sliding caliper and hence the pins on which the sliding caliper slides do not take any braking torque loads. However, both U.S. Pat. No. 4,200,173 and EP0752541 require an axially fixed structure on the outboard side of the brake disc in order to transfer the brake torque loads and axially slideable structure on the outboard side of the brake disc in order for the caliper to apply a clamp load to the brake pads when the brakes are applied. Thus, an appropriate space envelope must be provided for the axially fixed structure and the axially slideable structure on the outboard side of the brake disc where traditionally space is limited because of the proximity of the associated wheel rim.
EP0906856 shows an example of the braking torque loads from the outboard brake pad being transferred to the sliding caliper, but these braking torque loads are then transferred on to lugs of the fixed carrier which are positioned on the axially outboard side of the brake disc.
In U.S. Pat. No. 4,200,173, EP0906856 and EP0752541, because the brake torque loads are transferred via the circumferential ends of the pad brake backplate, an appropriate abutment must be provided on the carrier or the caliper in this region. This limits the design freedom of the caliper and the carrier.
A known problem with certain designs of brake calipers is that it is possible to fit the brake pads the wrong way, i.e., fit the brake pads such that the steel brake pad backplate faces the brake disc. Wrongly fitted brake pads are clearly unsafe. EP0752541 provides one way of ensuring the brake pads must be fitted the right way.